The present invention generally relates to thermopile-based thermal sensors. More particularly, this invention relates to a method and apparatus for performing self-testing of an infrared sensor.
A thermopile comprises a series of connected thermocouples, each made up of dissimilar electrically-resistive materials such as semiconductors and metals, and converts thermal energy into an electric voltage by a mechanism known as the Seebeck effect. The general structure and operational aspects of thermopiles are well known and therefore will not be discussed in any detail here.
Thermopiles have been employed in infrared sensors, a notable example being commonly-assigned U.S. Pat. No. 6,793,389 to Chavan et al., which discloses a thermopile transducer and signal processing circuitry combined on a single semiconductor substrate so that the transducer output signal (measured in volts) is sampled in close proximity by the processing circuit. The sensor comprises a frame surrounding a diaphragm on which the transducer is fabricated. The frame is formed of a semiconductor material that is not heavily doped, and signal processing circuitry is fabricated on the frame and electrically interconnected with the transducer so as to minimize signal noise. In particular, the close proximity between the transducer and the signal processing circuitry minimizes capacitive and inductive coupling to off-chip sources of electric and magnetic fields that would be potential sources of extraneous signals. Fabrication of the sensor structure does not require high dopant concentrations or thermal treatments that are incompatible with standard CMOS devices, such that the signal processing circuitry can make use of CMOS and BiCMOS technology. The sensor also does not require the use of materials and process steps that are not conducive to mass production processes made possible with CMOS and micromachining technology.
An optional feature of the sensor disclosed by Chavan et al. is the incorporation of a heating element that surrounds a central heat-absorption zone of the sensor diaphragm. For convenience, the heating element can be formed of polysilicon or another material deposited in the fabrication of the sensor or signal conditioning circuitry, the latter of which can be used to send current to the heating element to raise the temperature of the central heat-absorption zone of the diaphragm. This capability can be used as a self-test mechanism to determine if the transducer is functioning properly after packaging and installation in the field. By switching two different currents into the heating element, a change in transducer output voltage can be obtained that is proportional to the difference in the currents, or equivalently the generated heat in the diaphragm.
It would be desirable if a method were available for performing a wafer-level test on a thermopile-based infrared sensor of the type taught by Chavan et al., by which the sensor performance can be evaluated to identify sensors outside acceptable performance ranges. It would be particularly desirable if such a wafer-level test were suitable for high-volume testing of mass-produced sensors.